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My cat's flea treatment failed. What to do while waiting for new meds?

Updated On September 23rd, 2025

Pet's info: Cat | American Shorthair | Male | neutered | 2 years and 3 months old | 15 lbs

Hi, I gave my cat Hartz UltraGuard fleas and ticks. It didn’t work and fleas are growing in number. I was told that it is because my cat is large (15lbs) and also to stop Hartz because the treatment might be toxic. How long do I have to wait to give him advantage II for large cats? What can I do while I wait, since the fleas are still alive? Thank you!

4 Answers

Most Helpful Answer

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Answered By Dr. Strydom, DVM

Veterinarian

Published on May 7th, 2020

If you want an effective product please use one of newer ones like Cheristin. I don't think you will get any better results with Advantage. It is recommended to wait a minimum of 3 weeks between product applications. https://worldtails.securevetsource.com/itemView/view/198944_HomeDelivery.pml?productId=115569 While you wait you can get Capstar and give him that every couple days if need be. One pill lasts 24 hours but will kill everything on him. https://www.petco.com/shop/en/petcostore/product/capstar-flea-tablets-for-cats-2-25-lbs You also need to treat the house or the fleas will keep getting on him. I recommend Knockout Spray ES: https://www.chewy.com/virbac-knockout-es-area-treatment/dp/53856

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Other Answers

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    Answered By Dr. Melanie, BVSc MS

    Veterinarian

    Published on April 30th, 2019

    Since it's been two weeks, it's fine to give the Advantage now. You can also give her Capstar. It's a one time pill that starts killing fleas in 30 minutes. You will also have to disinfect the environment. Wash her bedding in hot soapy water, and vacuum the furniture and carpet surfaces. You can use a spray to kill fleas in the house, such as Indorex, or a flea bomb would work well to kill the fleas in the home. I hope this helps!

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    1Pet Parents found this answer helpful

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    Answered By Dr. Melanie, BVSc MS

    Veterinarian

    Published on July 18th, 2019

    Hartz is a pretty poor product, so I'm not surprised it didn't work. You have to wait 14 days before it's safe to apply the Advantage, so in two more days, you can do it. Keep giving Leon the Capstar until then. You will also have to disinfect the environment. Wash his bedding in hot soapy water, and vacuum the furniture and carpet surfaces. You can use a spray to kill fleas in the house, such as Indorex, or a flea bomb would work well to kill the fleas in the home. Best of luck.

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    1Pet Parents found this answer helpful

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    Answered By Dr. Melanie, BVSc MS

    Veterinarian

    Published on June 17th, 2019

    You are right that Hartz is pretty awful, and it's good that you washed if off Repo. Keep an eye out for drooling, agitation, and even seizures. If you see those signs, have her seen by a vet right away. I can give you a safe flea treatment program for her, just wait a few days before doing it. First, I would give her Capstar. It's a one time pill that starts killing fleas within 30 minutes. Then follow up with a spot on treatment, such as Advantage or Revolution. This is a treatment that has to be used once a month. You will also have to disinfect the environment. Wash her bedding in hot soapy water, and vacuum the furniture and carpet surfaces. You can use a spray to kill fleas in the house, such as Indorex, or a flea bomb would work well to kill the fleas in the home. I hope this helps!

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    1Pet Parents found this answer helpful

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